This invention relates to an improvement in an irradiation system with an ion beam, such as an ion implantation system. In the ion implantation system, an irradiation target is a wafer made of silicon on GaAs.
Following shrinkage of semiconductor devices, great importance has been placed on ion implantation accuracy even in a high-current ion implantation process. Conventionally, it has been sufficient that variation in the number (dose) of implanted ions is 1% or less in the surface of a target wafer and among targets wafer to wafer. However, in the most advanced process, it is required to control, in addition to the dose accuracy, ion implantation angle accuracy and energy accuracy at low energy. Factors that reduce the implantation angle accuracy are classified as follows.
(1) A center axis of an ion beam (hereinafter abbreviated as a “beam”) actually transported has an angle with respect to a designed center trajectory. This is called a center trajectory deviation.
(2) Inside a beam as a group of ions, variation in advancing direction of individual ions with respect to a center trajectory of the beam becomes large. This refers to an increase in beam divergence angle.
(3) In a batch ion implantation system, in the case of an implantation angle at the center of a target wafer is set at a value where a rotation axis of a disk mounted with the wafers thereon and the designed center axis of a beam are not parallel to each other, the implantation angles on the left and right sides of the wafer become different from a set value thereof due to rocking motion of the wafer with respect to the beam center axis. This is called an implantation angle deviation.
Among them, items (1) and (3) break fourfold asymmetry of a device character. This increases variation in device performance and extremely lowers yield of the most advanced devices. The beam divergence angle in item (2) serves to weaken the influence of items (1) and (3) when it is small. However, if the beam divergence angle is too large, distribution of dopants in a lateral direction is increased so that the size of a basic device structure become different from designed and therefore the device performance is degraded.
In order to solve the problem of item (1) in the batch ion implantation system, it is necessary to rotate the wafers and perform the ion implantation from four directions with respect to the designed beam axis. In this method, however, it is necessary to divide an otherwise one-time implantation operation into four-time implantation operations and turn all the ten-odd wafers by 90° for each implantation operation. Therefore, the productivity (throughput) is extremely lowered.
The problem of item (3) can be avoided by arranging the rotation axis of the disk and the beam center axis to be parallel to each other and by setting the wafer on the disk with a specific offset angle. However, there is no flexibility in implantation angle (tilt angle). Although it is possible to manufacture a disk with a small implantation angle deviation, as another solution for item (3), the wafer cooling performance of such a disk necessarily becomes insufficient. Therefore, in the batch high-current ion implantation system, it is impossible to enhance the accuracy of implantation angle without degrading the current performance of the system.
In addition to the above, the following points are cited as defects of the batch ion implantation system.
(A) It is not possible to carry out ion implantation at a large implantation angle exceeding 7°.
(B) Following high-speed rotation of the disk with the wafers mounted thereon, there is a possibility that a structure of the surface of each wafer is damaged due to particles moving on the wafer.
(C) Since it is invariably necessary to load ten-odd wafers per batch, a number of dummy wafers are required even when a small number of wafers are processed.
(D) Elastomer (rubber) is used as thermal conductor between a wafer and the cooled disk. Since the elastomer is easy to be pared by confacting a lot of wafers, a particle contamination amount is large on the back of each wafer.
The batch ion implantation system has a big merit that the beam utilization efficiency is high and the productivity is quite high. However, it can no longer be used as the high-current ion implantation system that is required to have high accuracy for coping with the device shrinkage.
Herein below, description will be given about first to third examples of a conventional single-wafer ion implantation system in place of the batch ion implantation system. The first example is a ribbon beam type single-wafer high-current ion implantation system, the second example is a two-dimensional mechanical scan type single-wafer high-current ion implantation system, and the third example is a beam scan type single-wafer middle-current ion implantation system. Note that the beam scan type single-wafer high-current ion implantation system which here we offer has not yet been proposed.
Referring to FIGS. 1 and 2, the ribbon beam type single-wafer high-current ion implantation system will be described.
In FIG. 1, a beam extracted from an ion source 101 through an extraction electrode 102 is subjected to a mass analysis in a dipole magnet 103. After the mass analysis, the beam diverges horizontally. The divergent beam passes through an acceleration/deceleration column electrode portion 104 and then is again focused and parallelized by a dipole magnet 105 also serving for parallelization and filtering, so as to be formed into a ribbon beam. The ribbon beam has a sheet-like cross-sectional shape and therefore is also called a sheet beam. At any rate, the ribbon beam has a beam section with a horizontal width greater than the wafer size and with a vertical width much smaller than the wafer size.
Here the direction of “horizontal” axis is defined as that is on the horizontal (dispersive) plane formed by the beam center trajectory in the mass-analysis magnet and that is perpendicular to the beam center trajectory. The “vertical” axis is perpendicular to both the horizontal plane and the beam center trajectory, The “longitudinal” axis is parallel to the beam center trajectory. Both the horizontal axis and the vertical axis are on the “transverse” plane. Although the scanning direction may different from the horizontal direction, for convenience, the scanning direction is regarded as the horizontal direction in the following description.
Ion implantation is carried out by fixing a beam while mechanically moving a wafer upward and downward by the use of a platen 106. A low-energy beam is obtained by extracting and transporting a beam at high energy and decelerating it at a deceleration stage 107 installed near the wafer.
The ion implantation system of the first example has the following advantages.
The space-charge effect (repulsion between ions) causing a reduction in low-energy beam current is proportional to a beam density. Since the ribbon beam has a large cross-sectional area, insofar as the beam current is the same, the beam density becomes lower as compared with the other types. On the other hand, if the transportable beam density is the same, the beam current becomes larger.
However, the ion implantation system of the first example has the following defects.
Uniformity of the beam density in the horizontal direction, as it is, represents dose uniformity in the horizontal direction. It is quite difficult to achieve a beam density non-uniformity of 1% or less. Particularly at low energy, the beam diverges naturally due to the space-charge effect and therefore it is almost impossible to control the beam density.
Further, since the beam is decelerated near the wafer, beam ions neutralized before the deceleration due to interaction with a plasma shower gas or a resist out-gas are implanted as they are without being decelerated. Such beam ions become energy contamination. Further, since such beam ions are not measured as beam current, there also occurs an error in dose (overdose).
Further, those ions, that are neutralized due to collision with the gas while the beam is passing through the parallelizing electromagnet 105 become uncertain in deflection angle and are therefore implanted into a wafer 108 at abnormal implantation angles as shown in FIG. 2 by arrows. This is caused by the fact that the sheet plane of the ribbon beam and the deflection plane (the beam trajectory plane in the parallelizing electromagnet 105) coincide with each other in the horizontal plane.
After all, like the foregoing batch ion implantation system, the ribbon beam type single-wafer high-current ion implantation system is high in productivity because its beam current is large, but poor in implantation accuracy.
Referring to FIG. 3, the two-dimensional mechanical scan type high-current ion implantation system will be described.
In FIG. 3, a beam extracted from an ion source 201 through an extraction electrode 202 is subjected to a mass analysis in a mass analysis electromagnet device 203. After the mass analysis, a wafer on a platen 206 is irradiated with the beam through a differential lens 205. In this example, the beam is fixed and ion implantation is performed over the whole surface of the wafer by mechanical scanning, i.e. by mechanically moving the wafer vertically and horizontally by the use of the platen 206. The cross-sectional size of the beam is much smaller than the wafer size in both vertical and horizontal directions. In the case of extremely low energy implantation, a decelerated beam is used.
The ion implantation system of the second example has the following advantages.
A relatively large amount of the beam current is obtained at low energy.
A beam line from the ion source 201 to the platen 206 is short and the ion implantation system is offered at a relatively low price.
However, the ion implantation system of the second example has the following defects.
The defects inherent to the beam line of the batch ion implantation system, such as the beam axis deviation, the increase in beam divergence angle at low energy, and the generation of energy contamination in deceleration of the beam, are inherited as they are.
Since a scanning frequency cannot be set high but can only be set to about 1 Hz in the mechanical scanning, the scan times that the beam passes through each point on the wafer per unit time is small. In order to suppress no uniformity of dose in the surface of the wafer to 1% or less, the scan times should be set to about 100. In order to achieve it in a system using a low scanning frequency, a beam should be intentionally reduced so as to prolong an implantation time. That is, the productivity should be sacrificed for enhancing the dose accuracy.
After all, the two-dimensional mechanical scan type high-current ion implantation system is low in productivity and also not good in implantation angle accuracy.
Referring to FIGS. 4A and 4B, the beam scan type middle-current ion implantation system will be described. Top view of the system is shown in FIG. 4A and side view is in FIG. 4B. Such a middle-current ion implantation system is disclosed in, for example, JP-A-2003-288857.
In FIG. 4A, ions generated in an ion source 301 are extracted as an ion beam 302 through an extraction electrode (not illustrated). The extracted beam 302 is subjected to a mass analysis in a dipole electromagnet 303 so that only a necessary ion species is selected. The ion beam 302 composed of only the necessary ion species is supplied into a beam transformer 304 where the cross-sectional shape of the beam 302 is adjusted to the beam transportation line. The beam transformer 304 is formed by a magnetic Q (Quadrupole) lens, an electrostatic Q (Quadrupole) lens, or the like. The beam having the appropriate cross-sectional shape is deflected by a scanner 305 in a plane parallel to the sheet surface of FIG. 4A.
The scanning beam is parallelized by a parallelizing-lens (hereinafter refered to as “P-lens”) 306 so as to be parallel to an axis of a deflection angle of 0°. In FIG. 4A, a scan range of the beam by the scanner 305 is indicated by thick black lines and a broken line. The beam from the P-lens 306 is sent to an angular energy filter 308 (hereinafter also referred to as an “AEF”) through one or more acceleration/deceleration column electrodes 307. The angular energy filter 308 performs an analysis about energy of the ion to thereby select only an ion species with necessary energy. As shown in FIG. 4B, only the selected ion species is deflected slightly downward in the angular energy filter 308. The beam composed of only the thus selected ion species is transported to a wafer 310 through a plasma electron flood system 309. The beam that is not injected to the wafer 310 is incident on a beam stopper 311 so that energy thereof is consumed. Normally, the structure from the ion source 301 to a vacuum process chamber where the wafer 310 is accommodated is called a beam line.
In this type of the ion implantation system, the beam extracted from the ion source 301 is, after the mass analysis, deflected horizontally at a scanning frequency of several hundreds of Hz to several KHz by the scanner 305 and then parallelized by the P-lens 306. The cross-sectional size of the beam is much smaller than the size of the wafer 310 and the beam scan range in the horizontal direction is set greater than the wafer 310. With respect to the vertical direction, mechanical scanning is implemented to move the wafer 310 like in the ribbon beam type. After the parallelization, the beam is accelerated or decelerated by the acceleration/deceleration column electrode or electrodes 307 so as to cover a wide energy range of 5 keV to 260 keV. By performing the energy analysis through the angular energy filter 308 in the form of an electric field or a magnetic field after the acceleration or deceleration, the pure beam can be implanted into the wafer 310. Although not illustrated, energy slits are installed on the downstream side of the angular energy filter 308.
The ion implantation system of the third example has the following advantages.
The beam parallelism can be measured so that the implantation angle in the horizontal direction can be set uniformly over a wafer with high accuracy.
Since the scanning frequency is high, high dose uniformity can be achieved in the wafer even with the implantation for a short time.
By the use of the angular energy filter 308, all the energy contamination and the major part of particle and metal contamination generated in the beam line are prevented from going toward the wafer.
The parallelization of the beam and the energy filtering for the beam are performed by the different devices so that the beam scan plane (horizontal) and the deflection trajectory plane vertical of the angular energy filter 308 can be set perpendicular to each other. Therefore, a part of the beam that is neutralized while passing through the angular energy filter 308 and coming out from it at abnormal angles cannot pass through the narrow slits in front of a wafer and is therefore prevented from being implanted into the wafer 310.
However, the ion implantation system of the third example has the following defects.
The amount of transportable beam current is small. Particularly at extremely low energy, the beam can hardly be transported.
After all, the beam scan type middle-current ion implantation system enables highly accurate implantation but cannot transport to the wafer a beam current high enough for use in the high-current ion implantation process.